LEDs are known to act as a source of emitted light for a wide variety of applications. LEDs are known to provide many advantages over incandescent and fluorescent illumination because of their long operating life, high efficiency, lightweight, and low profile.
FIG. 4 is a schematic diagram illustrating a conventional LED display including an LED driver circuit 50 for driving an LED chain 52 made up of serial connected LEDs 53-1 to 53-N. LED driver circuit 50 includes a DC-DC boost converter 51 that is utilized in conjunction with a passive inductor LEXT and a passive discharge capacitor CEXT. DC-DC boost converter 51 includes a comparator (operational amplifier) 55, a pulse width modulation (“PWM”) circuit 58, an internal capacitor CINT, an npn bipolar transistor T1 and a zener diode S1. An external voltage (VIN) is supplied, for example, from a battery, to the anode of diode S1 and to the collector of transistor T1 by way of inductor LEXT. The voltage level at the output node 54, which is connected to a first end of the LED chain 52, is established by a user supplied reference voltage “VREF” applied to the input terminal (node 57) that is connected to the inverting input terminal of comparator 105, also sometimes referred to in the art as the “error amplifier”. The second end of LED chain 52 is connected to the non-inverting input terminal of comparator 55, which is also connected to ground by way of an appropriately sized external resistor “REXT” 56. The output voltage at node 54 adjusts until the loop through boost converter 51 controls the current passing through LED chain 52 such that the current is defined as I=VREF/REXT, where VREF is a regulated voltage powered from VIN. Light output from LED chain 52 is proportional to the current generated by LED driver 50, and can therefore be selectively increased by way of increasing the current generated by LED driver circuit 50. LED driver circuits similar to those shown in FIG. 4 are used in commercial products such as the Model 2287 integrated circuit manufactured by the assignee of the present application, that drive LED chains such that current feedback substantially constantly adjusts the power to the LED chain (see also, e.g., Min et al., U.S. Pat. No. 6,586,890, which is incorporated herein by reference in its entirety).
LEDs have two key characteristics: color and brightness. In LED lighting applications it is often desirable to change the brightness of the light emitted while maintaining a fixed color. Because the color and brightness are both controlled by the current passing through the LED, the desire to change brightness conflicts with producing a fixed color. Designers can circumvent this problem by maintaining a constant current through the LED and turning it ON/OFF very quickly using a PWM circuit, such as that depicted in FIG. 4. By changing the ratio of ON time to OFF time, while keeping the same pulse repetition rate, the perception of varying brightness can be achieved. When PWM ON/OFF control is used, the pulse repetition rate is called PWM frequency, and the ratio of ON time to the pulse repetition period is called duty cycle.
The human eye's ability to average or smooth this stream of light pulses into what appears to be a steady light depends upon the rate at which the LED switches ON/OFF. The shorter the time between light pulses, the more constant and unwavering the light appears to be. To achieve a wide range of brightness control, the minimum duty cycle will typically be 1%. Also, to avoid the perception of flicker (being able to detect the individual pulses), the PWM frequency should ideally be 1 kHz or greater. This pulse width shrinks more as either the PWM frequency or the duty cycle is increased. Because a typical circuit that is used to control current or voltage has a finite response time, the combination of high PWM frequency and low duty cycle can go beyond its ability to keep the current (or voltage) constant. This means that, the higher the frequency and the lower the duty cycle of the PWM control signal, the harder to adjust a LED's brightness while keeping its color constant.
What is needed is a LED driver circuit that avoids the brightness/color problems associated with conventional LED driver circuits.